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Photoinduced Multistage Phase Transitions in Ta2nise5

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Photoinduced Multistage Phase Transitions in Ta2nise5 Photoinduced multistage phase transitions in Ta2NiSe5 ‡ 1 ‡ 1,2 ‡ 3 1 1 1 Q. M. Liu , D.Wu ∗ , Z. A. Li , L. Y. Shi, Z. X. Wang, S. J. Zhang, 1 1 3 3 1 1, 4 T. Lin, T. C. Hu, H. F. Tian, J. Q. Li, T. Dong, and N. L. Wang∗ 1 International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China 2 Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China 3 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4 Collaborative Innovation Center of Quantum Matter, Beijing, China Abstract: Utrafast control of material physical properties represents a rapid developing field in con- densed matter physics. Yet, accessing to the long-lived photoinduced electronic states is still in its early stage, especially with respect to an insulator to metal phase transition. Here, by combing transport mea- surement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits the lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy reveals an in-chain Ta atom displacement in the photoinduced new structure phase. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition. ∗ Corresponding author: [email protected]; [email protected] Ultrashort laser pulse not are only a powerful tool to excite and probe non-equilibrium electronic processes in transient states, but also emerges as an useful method to induce phase transitions that may or may not be thermally accessible. The latter has drawn increasing attention because it enables ultrafast optical manipulation and control over material properties [1–13]. Among various photoinduced (PI) phase transitions and related phenomena, the ultrafast switching from an insulating to a metallic state is particularly attractive, for it has high potential for device applications. While some phase transitions are transient, in the sense that they recover on very short time scales, e.g. picoseconds (ps), others are permanent or meta-stable and require something other than time to reset them. Up to now, the PI insulator to steady metallic state is achieved only in few systems exhibiting competing orders or broken symmetry phases, e.g. manganites perovskites [3, 4, 9, 13] and a charge density wave (CDW) crystal of 1T-TaS2[8], with their PI states only surviving at temperatures far below the room temperature. PI nonthermal insulator-metal phase transition was also reported in a strongly correlated material VO2 [14, 15], however, a recent study questioned the claim and found no evidence for a hidden transient Mott-Hubbard non-thermal phase in VO2 [16]. Here we report on distinct PI multistage phase transitions with insulator-to-metal transition (IMT) characteristics in Ta2NiSe5 -the compound is currently known as an important candidate of excitonic insulator (EI) [17–19]. We show that in Ta2NiSe5, the new steady phase induced by the strong laser pulses has a lattice structure different from any of the initial equilibrium states and is thermally stable up to at least 350 K. Ta2NiSe5 is a two-dimensional (2D) van der Waals compound with typical in-plane quasi-one-dimensional (1D) substructure composed of parallel Ta and Ni atomic chains along a-axis of the lattice (Fig. 1a). Upon cooling, the system shows a second-order phase transition at Tc = 326 K accompanied by a slight shear distortion of those atomic 1D chains, that reduces the lattice symmetry from orthorhombic to monoclinic [19]. Our start point is the observation of the sharp resistance drop of ultrathin Ta2NiSe5 crystals induced by ultrafast laser pulses. The samples, with thicknesses of ∼ 30 nm (less than the penetration depth ∼ 55 nm at λ = 800 nm), are exfoliated from the bulk crystals (See Methods. Images of typical devices are shown in Supplementary Figure 1). To induce the resistance switching, laser pulses from a Ti:sapphire amplifier system with 800 nm wavelength, 35 fs duration and 2 1 kHz repetition were firstly used as the “writing” pulses with a fluence of ∼ 3.5 mJ/cm . As shown in Fig. 1b, after writing pulses, the resistance drops approximately four orders of magnitude at 50 K thereafter retains in a low resistance (LR) state. This PI-LR state is thermally stable up to at least 350 K by the measurement. Unlike the pristine resistivity showing a transition at Tc of 326 K, no anomaly is indicated in the temperature dependent resistivity down to 1.8 K for PI-LR state (Supplementary Figure 2). The activation energy of the PI-LR state is tiny and estimated as 4.3 meV, twenty times smaller than 2 the one of pristine state. To induce the PI-LR state, a threshold fluence ∼ 2.5 mJ/cm is required. We remark that, due to the very high resistance at low temperature, we could not measure the value of the resistance for the pristine nano-thickness sample below 50 K. It is estimated that the resistance can drop by 8 orders at 4 K by such ultrafast laser pulse excitations. The PI-LR state is repeated from device to device, no matter the sample growth batches. To gain insight into the microscopic nature of the PI-LR state, we investigated the time-resolved photo-excitations by pump-probe spectroscopy. In the experiments, the energies of pump and probe 2 2 pulse are kept low at ∼ 50 µJ/cm and ∼ 3 µJ/ cm respectively, to ensure minimal disturbance of those states (see Methods). Fig. 2a depicts the photoinduced relative change of transient reflectivity ΔR/R(t). The rising time is about 100 fs. It can be clearly recognized that the decay process of photoinduced reflectivity change ΔR/R(t) is superimposed by several coherent oscillations [20]. The reflectivity change could be well reproduced by two-exponential decay processes[21, 22], a fast one in the time scale of 0.5 -0.6 ps and a slow one with ∼ 10 ps. Presence of rapid and slow decay dynamics after FIG. 1. Resistivity switching of Ta2NiSe5 by the 35-fs laser pulse at 800 nm. a, The layered crystal structure of Ta2NiSe5. A quasi one-dimensional structure is formed by a single Ni chain and two Ta chains along a-axis. b, The four-probe resistance of the pristine (green) and PI-LR state (pink) after a 2 writing excitation ∼3.5 mJ/cm . Inset is the schematic of the sample and experimental configuration. The length of dotted red line indicates the time delay between the pump and probe pulses excitation has been observed in many systems. In general, the number of photoexcited hot electrons at zero time delay is related to the amplitude of ΔR/R. Those excited high-energy hot electrons release and transfer their energy to lattice through the emission of optical phonons. The sub-picosecond decay would be mainly attributed to hot electron relaxation via the energy exchange with strongly coupled phonons, the ∼ 10 ps decay process would be related to the energy exchange with other phonons or dephasing process. The fast decay process is in accordance well with the electronic pre-thermalization process revealed by tr-ARPES experiments [23, 24]. The coherent oscillations arise from the coherent phonon excitations. We extract the coherent phonon spectra by fast Fourier transformation from transient reflectivity spectra after subtracting the electronic decay background, as shown in Fig. 2b. For pristine state at 50K, five phonon modes of 1.0-, 2.0-, 3.0-, 3.7-, 4.0-THz are detected. These modes are identified as Ag symmetries in earlier studies [25], arising from displacive excitation of coherent phonons [26]. Remarkable differences can be found between the spectra of pristine state and PI-LR state at low temperatures. As shown in Fig.2b, the 2.0-and 3.7-THz modes are absent for PI-LR state at 50K. Besides, the peak frequencies of the remaining modes of PI-LR state are a little higher comparing with the ones of pristine state respectively. These changes in phonon FIG. 2. Pump-probe response spectra. a, Transient photoinduced reflectivity at various states. The signal is made up of the electronic response and the coherent oscillations. The gray lines represent fits to the measured data. b, the corresponding FFTs from transient reflectivity spectra after substracting the incoherent components. c, the temperature evolution of the coherent phonon spectra of pristine state (upper panel) and the PI-LR state (lower panel). In the experiment, the light polarizations of pump and probe are set parallel and perpendicular to a-axis respectively. number and frequency imply a PI lattice structural phase transition. More information of coherent modes evolution are recognized from the temperature dependent spectra (Fig. 2c). For the spectra of pristine sample, the 2.0 THz mode is present solely in the low temperature monoclinic phase, and the modes 3.7-and 4.0 THz gradually merge into one 3.7-THz across Tc accompanying the structural transition from monoclinic to orthorhombic phase. While for the PI-LR state, the temperature-dependent spectra show only three recognizable modes throughout the measured temperature range, irrespective of thermal cycles. Although the PI-LR phase does not show a phase transition in the measured temperature range, the coherent phonon modes in the PI-LR phase are very similar to those in the high temperature pristine phase in Ta2NiSe5, therefore, it is expected that the PI-LR phase is in the similar lattice symmetry but with different atomic construction comparing to the pristine high temperature orthorhombic phase.
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